Most Helpful Guy
Some minority of men really struggle with monogamy, so cheat continually. There can be a few reasons behind that.
A larger group of men will eventually cheat in marriage if the marriage relationship is basically just 'raise kids together', and then at some point some other woman shows a genuine sexual desire for him, when he felt unwanted for so long.
Most cheating is done by the first group, since they cheat all the time, so women run into them a lot. But if you look at all married men who cheat, more would be the second type.
Sometimes the second type becomes more like the first, realizing that their spouse doesn't want them, so they fill their marriage with affairs proactively.2